1. Field of the Invention
The present invention related to a display control apparatus that executes gamma correction of a display device such as a liquid crystal panel. In particular, the invention relates to a display control apparatus that executes gamma correction using a look-up table, and a method of creating a look-up table used for the gamma correction.
2. Description of Related Art
In the case of displaying an image on a display device such as a liquid crystal panel, grayscales of image data to be displayed should be corrected in accordance with gamma characteristics of the display device. Such correction is called “gamma correction” Incidentally, a correcting method using a look-up table (LUT) has been conventionally and widely used for gamma correction of digital image data (for instance, Japanese Unexamined Patent Publication Nos. 2001-238227 and 7-56545).
The LUT is a table that stores grayscale values of digital image data after being subjected to the gamma correction in association with grayscale values of the digital image data. FIG. 16 shows an example of the LUT. The LUT of FIG. 16 is an LUT for gamma correction that converts image data of 64 grayscales (6 bits) into image data of 256 grayscales (8 bits), and an LUT address represents a grayscale value of image data before the gamma correction, and an LUT value represents a grayscale value after the gamma correction.
In this way, the LUT realizes one-dimensional array by storing grayscale values of image data after gamma correction with grayscale values of the image data before the gamma correction used as an argument. Accordingly, if an LUT is stored in a memory having as many address bus width as the number of bits of the image data before gamma correction and as manγ data bus width as the number of bits of the image data after the gamma correction, and the above LUT address is used as an input address, an output of the LUT value can be obtained.
Further, it is necessary to switch a current LUT to an appropriate LUT in accordance with whether or not to execute gamma correction on input image data, and changes in surrounding environments and image type instead of executing gamma correction for one display device by use of one LUT all the time. Hence, a display control apparatus that switches LUTs to execute suitable gamma correction has been hitherto known.
FIG. 15 shows an example of a conventional display control apparatus. A display control apparatus 80 is a controller driver for receiving image data from a processor 2 such as a CPU to display an image on a liquid crystal panel 4. A control circuit 81 receives image data and LUT data from the processor 2. A grayscale converting circuit 14 executes gamma correction, which references the LUT to convert grayscales of input image data. Further, the grayscale converting circuit 14 outputs converted image data to a data line driving circuit 16. The data line driving circuit 16 applies a voltage that is selected from grayscale voltages generated by a grayscale voltage generating circuit 15 in accordance with the image data, to the liquid crystal panel 4. A gate line driving circuit 3 applies a gate pulse to the liquid crystal panel 4 in accordance with a driving timing control signal output from the control circuit 81 to drive the liquid crystal panel 4.
A LUT memory 90 stores plural different LUTs. When the grayscale converting circuit 14 changes an LUT used for the gamma correction, the processor 2 outputs the LUT data read from the LUT memory 90 to the control circuit 81, and the grayscale converting circuit 14 receives the LUT data from the control circuit 81 to update the LUT.
The above conventional display device faces a problem in that a large memory capacity is necessary for storing an LUT. For example, provided that the number of bits of image data before gamma correction is i, that is, the number of grayscales is 2i, and the number of bits of the image data after the gamma correction is k, that is, the number of grayscales is 2k, an LUT data size necessary for the grayscale conversion equals (2i*k bits). Considering that the image data before gamma correction is 6-bit data, and the corrected image data is 8-bit data, the LUT data size equals 512 bits (=26*8).
As the number of bits of image data increases, the LUT data size exponentially increases. A memory capacity necessary for storing the LUT is accordingly increased. Incidentally, image data of R (red), G (green), and B (blue) have different gamma characteristics, so LUTs have to be provided for each of R, G, and B. Further, in the case of switching the LUTs in accordance with surrounding circumstances, the above LUT memory 90 stores plural LUTs in accordance with the surrounding circumstances, and thus requires a large memory capacity. As a LUT data size increases, it takes longer time to transfer LUT data from the processor 2 to the display control apparatus 80, and to update an LUT of the grayscale converting circuit 14.